1. Field of the Invention
The present invention relates to a control apparatus for controlling a plant which has non-linear characteristics such as hysteresis, dead band and the like.
2. Description of the Prior Art
A known control apparatus of the type mentioned above is disclosed, for example, in Laid-open Japanese Patent Application No. 2001-132482. This control apparatus controls a cam phase varying mechanism as a plant. The cam phase varying mechanism freely varies the phase relative to a crank shaft (hereinafter called the “cam phase”) of an intake cam in an internal combustion engine, and is hydraulically driven by an oil pressure supplied from an oil pump. Also, the control apparatus comprises a crank angle sensor and a cam angle sensor for detecting signals corresponding to angular positions of the crank shaft and the intake cam, respectively, and a controller which receives detection signals of these sensors.
This controller calculates an actual cam phase based on the detection signals of the crank angle sensor and the cam angle sensor, calculates a target cam phase based on the operating condition of the internal combustion engine, and calculates a control input to the cam phase varying mechanism in accordance with a sliding mode control algorithm to control the cam phase to converge to the target cam phase.
It is generally known that the above-mentioned hydraulically driven cam phase varying mechanism, when regarded as a plant, has strong non-linear characteristics such as hysteresis, dead band and the like. In contrast, the control apparatus described in Laid-open Japanese Patent Application No. 2001-132482 calculates a control input in accordance with the sliding mode control apparatus, so that when this control apparatus controls a plant having strong non-linear characteristics, i.e., a hydraulically driven cam phase varying mechanism, the control apparatus fails to control the cam phase with the control input in small changing steps, due to the strong non-linear characteristics, and therefore suffers from problems of a low resolution of control and a low accuracy of control.
The present applicant has proposed a control apparatus which can solve the problems of the control apparatus as described above, in Laid-open Japanese Patent Application No. 2005-63177. This control apparatus controls an electromagnetically driven cam phase varying mechanism, and comprises a two-degree-of-freedom sliding mode controller and a DSM controller. The two-degree-of-freedom sliding mode controller calculates a control value for a cam phase to converge to a target com phase in accordance with a target value filter type two-degree-of-freedom sliding mode control algorithm. The DSM controller in turn modulates the calculated control value in accordance with an algorithm to which a ΔΣ modulation algorithm is applied to calculate a control input to the cam phase varying mechanism such that the control input frequently repeats inversions over a predetermined amplitude centered at a predetermined value. As a result, even for controlling a cam phase varying mechanism having strong non-linear characteristics, the control apparatus can control the cam phase in small changing steps in response to the control input, which frequently repeats inversions, while compensating for the non-linear characteristics to improve the resolution of the control.
The cam phase varying mechanism has the nature of varying the non-linear characteristics as an internal combustion engine varies in the operating condition. Particularly, when the cam phase is changed, the non-linear characteristics tend to vary because the cam phase varying mechanism is affected by a cam counter-force and sprocket fluctuations (i.e., fluctuations in chain speed or fluctuations in crank angular speed). For example, as the cam counter-force or the sprocket fluctuations become larger, the cam counter-force or the sprocket fluctuations themselves act to change the cam phase, often resulting in such a change that increases the sensitivity of the cam phase to the control input when the cam phase is changed. If the internal combustion engine becomes instable in combustion condition in the manner mentioned above, a change in the cam counter-force or the sprocket fluctuations would cause a change in the sensitivity of the cam phase to the control input in the variable cam phase mechanism. Also, when an oil pressure is supplied from a hydraulic pump which is driven by the torque of the internal combustion engine in the hydraulically driven cam phase varying mechanism, fluctuations in the rotational speed of the engine would cause a change in the oil pressure supplied to the cam phase varying mechanism, thereby leading to changes in the sensitivity of the cam phase to the control input and in the frequency stability of the control input as well as an additional change in the non-linear characteristics.
When an attempt is made to compensate the cam phase varying mechanism as described above for a change in the non-linear characteristics, the control apparatus described in Laid-open Japanese Patent Application No. 2005-63177 can compensate for the change in the non-linearity by setting the amplitude of the control input to a larger value under conditions in which the non-linear characteristics present a larger change. In this strategy, however, the inverted state of the control input is reflected to the cam phase as a control amount just like noise under conditions in which the sensitivity of the control amount is degraded with respect to the control input, particularly under conditions in which the frequency sensitivity is degraded, more specifically under conditions in which a high frequency shut-off characteristic is degraded, possibly resulting in a lower resolution of control, to the contrary, and a lower accuracy of control.